Imaging the growth and retraction of dendritic spines has revealed that synapses can be gained and lost in an experience-dependent manner. In vitro, proteins in the postsynaptic density (PSD), including receptors and scaffolds, exhibit activity-dependent trafficking and turnover that relate to synapse and spine growth, with these dynamics partially determining synaptic persistence and strength. The goal of this proposal is to image select populations of PSD components tagged with photoactivatable derivatives of GFP to measure protein trafficking dynamics in vivo and examine PSD assembly and turnover. The specific aims include: (1) Characterize chimeric postsynaptic proteins tagged with photoactivatable GFP in brain slice cultures, (2) Document the basal dynamics of PSD component trafficking and turnover in vivo in developing mice, and (3) To measure the effect of experience-dependent plasticity on PSD component trafficking in the developing neocortex in vivo. Understanding the role of individual proteins in the modifications of complex circuits in vivo would be a significant step forward in neuroscience, bridging the gap between the cell biology of neuronal culture systems and the plasticity of in vivo circuit remodeling. [unreadable] [unreadable]